Semiconductor device

ABSTRACT

A conventional DRAM needs to be refreshed at an interval of several tens of milliseconds to hold data, which results in large power consumption. In addition, a transistor therein is frequently turned on and off; thus, deterioration of the transistor is also a problem. These problems become significant as the memory capacity increases and transistor miniaturization advances. Another problem is that an increase in memory capacity leads to an increase in the area, despite an attempt at integration through advancement of transistor miniaturization. A transistor is provided which includes an oxide semiconductor and has a trench structure including a trench for a gate electrode and a trench for element isolation. In addition, a plurality of memory elements each including the transistor having a trench structure and including an oxide semiconductor is stacked in a semiconductor device, whereby the circuit area of the semiconductor device can be reduced.

TECHNICAL FIELD

The present invention relates to a technique for miniaturizing semiconductor integrated circuits. The invention disclosed in this specification includes in its scope an element formed using a compound semiconductor, in addition to that formed using a silicon semiconductor, as a component of a semiconductor integrated circuit, and discloses an element formed using an oxide semiconductor as an example.

BACKGROUND ART

As semiconductor memory devices, dynamic RAMs (DRAMs) are well-known products and currently used in a variety of electronic devices. A memory cell which is a key component in a DRAM includes a read and write transistor and a capacitor.

Circuit patterns for DRAMs, like those for other semiconductor integrated circuits, have been miniaturized in accordance with the scaling law, and there was a time when it was considered difficult to achieve a design rule of 100 nm or less. One of the reasons is that in a transistor having a channel length of 100 nm or less, a punch-through current is likely to flow due to a short-channel effect and the transistor becomes incapable of functioning as a switching element, which has been considered to be a problem. In order to prevent a punch-through current, a silicon substrate may be doped with an impurity at high concentration. However, this is not an appropriate solution to the problem because it makes a junction leakage current likely to flow between a source and the substrate or between a drain and the substrate and eventually causes a deterioration of memory retention characteristics.

Against such a problem, a method has been considered for reducing the area occupied by one memory cell and also maintaining an effective channel length so as not to cause a short-channel effect by forming a three-dimensional transistor in the memory cell. One example is a structure in which a U-shaped vertically long groove is formed in a region where a channel portion of a transistor is formed, a gate insulating film is formed along a wall surface in the groove, and a gate electrode is formed so as to fill the groove (see Reference 1).

A transistor having a channel portion of such a structure has a long effective channel length because a current flows between a source region and a drain region via an indirect route across the groove portion. This provides an advantageous effect of reducing the area occupied by a transistor in a memory cell and suppressing a short-channel effect.

REFERENCE

-   [Reference 1] Kinam Kim, “Technology for sub-50 nm DRAM and NAND     Flash Manufacturing”, International Electron Devices Meeting 2005,     IEDM Technical Digest, December 2005, pp. 333-336

DISCLOSURE OF INVENTION

However, a conventional DRAM needs to be refreshed at an interval of several tens of milliseconds to hold data, which results in large power consumption. In addition, a transistor therein is frequently turned on and off; thus, deterioration of the transistor is also a problem. These problems become significant as the memory capacity increases and transistor miniaturization advances.

Another problem is an increase in the area of a semiconductor device. There is a strong demand for memory devices with larger memory capacity. The increase in memory capacity leads to an increase in the area, despite an attempt at integration through advancement of transistor miniaturization.

Thus, it is an object of the present invention to provide a technique that can improve data retention characteristics of a semiconductor memory device. Another object is to provide a technique that can reduce power consumption as well as improving data retention characteristics of a semiconductor memory device. It is still another object to provide a technique that can decrease the area of a semiconductor device as well as improving data retention characteristics and reducing power consumption of the semiconductor device.

In order to achieve any of the above objects, a circuit, specifically a semiconductor memory device, is formed using a transistor including an oxide semiconductor (OS), particularly a MOS transistor including an oxide semiconductor. The oxide semiconductor is a substantially intrinsic semiconductor. For that reason, there is the advantage that the off-state current is extremely small.

With the use of the transistor including an oxide semiconductor, the interval between refresh operations can be longer than that for a conventional DRAM, and power consumption can be reduced. In addition, the number of times a transistor is turned on and off per unit time can be reduced, and therefore, the lifetime of the transistor can be made longer than that in a conventional DRAM.

Even in a transistor including an oxide semiconductor layer, a short-channel effect might be caused with the advancement of transistor miniaturization. In view of this, a novel transistor structure including an oxide semiconductor layer is proposed. In addition, a plurality of memory elements each including the novel transistor including an oxide semiconductor layer is stacked in a semiconductor device, whereby the circuit area of the semiconductor device can be reduced. Details thereof will be described below.

One embodiment of the present invention is a semiconductor device which includes a first transistor including a material other than an oxide semiconductor, a peripheral circuit including the first transistor, a second transistor including an oxide semiconductor, a capacitor electrically connected to the second transistor, and a memory element including the second transistor and the capacitor. The second transistor includes a first trench and a second trench in an insulating layer, an oxide semiconductor layer in contact with a bottom surface and an inner wall surface of the first trench, a gate insulating layer over the oxide semiconductor layer, a gate electrode over the gate insulating layer, the gate insulating layer over a bottom surface and an inner wall surface of the second trench, an insulating layer filling the second trench, and a source electrode or a drain electrode in contact with the oxide semiconductor layer. The memory element is stacked over the peripheral circuit. The first trench is a trench for the gate electrode, and the second trench is a trench for element isolation. Note that an upper surface shape of the first trench is a stripe shape or a rod-like shape, and an upper surface shape of the second trench is a lattice shape, a stripe shape, or a rod-like shape.

In the above structure, the gate insulating layer may be in contact with and cover a side surface of the oxide semiconductor layer. The oxide semiconductor layer may include a crystal having a c-axis substantially perpendicular to a surface of the oxide semiconductor layer. The material other than the oxide semiconductor may include silicon, germanium, silicon germanium, silicon carbide, or gallium arsenide.

The thickness of the oxide semiconductor layer is preferably greater than or equal to 1 nm and less than or equal to 100 nm, and the oxide semiconductor layer may be a crystalline oxide semiconductor layer. With the crystalline oxide semiconductor layer, it is possible to provide a highly reliable semiconductor device in which changes of the electrical characteristics of the transistor due to irradiation with visible light or ultraviolet light can be suppressed. Further, the crystalline oxide semiconductor layer includes an oxide including a crystal with c-axis alignment (also referred to as c-axis aligned crystal (CAAC)), which has neither a single crystal structure nor an amorphous structure.

A technique that can improve data retention characteristics of a semiconductor memory device can be provided.

A technique that can reduce power consumption as well as improving data retention characteristics of a semiconductor memory device can also be provided.

A technique that can decrease the area of a semiconductor device as well as improving data retention characteristics and reducing power consumption of the semiconductor device can also be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are cross-sectional views and a top view of one embodiment of the present invention.

FIGS. 2A to 2C are cross-sectional schematic views of one embodiment of the present invention.

FIGS. 3A and 3B are a cross-sectional view and a circuit diagram of one embodiment of the present invention.

FIG. 4 is a cross-sectional view of a semiconductor device of one embodiment of the present invention.

FIGS. 5A and 5B are a circuit diagram and a conceptual diagram of a semiconductor device of one embodiment of the present invention.

FIG. 6 is a cross-sectional view of a semiconductor device of one embodiment of the present invention.

FIGS. 7A and 7B are a cross-sectional view of a structure used for calculation and results of the calculation.

FIGS. 8A and 8B are a cross-sectional view of a structure used for calculation and results of the calculation.

FIGS. 9A and 9B are a cross-sectional view of a structure used for calculation and results of the calculation.

FIGS. 10A and 10B are circuit diagrams of semiconductor devices.

FIG. 11 is a diagram illustrating a portable device including a semiconductor device.

FIG. 12 is a diagram illustrating a portable device including a semiconductor device.

FIG. 13 is a diagram illustrating a portable device including a semiconductor device.

FIGS. 14A to 14E illustrate structures of oxide materials according to one embodiment of the present invention.

FIGS. 15A to 15C illustrate a structure of an oxide material according to one embodiment of the present invention.

FIGS. 16A to 16C illustrate a structure of an oxide material according to one embodiment of the present invention.

FIG. 17 is a cross-sectional view of one embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in detail below with reference to drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details of the present invention can be modified in various ways. In addition, the present invention should not be construed as being limited to the description in the embodiments given below.

Embodiment 1

In this embodiment, a structure of a transistor and a method for manufacturing the transistor according to one embodiment of the present invention will be described with reference to FIGS. 1A to 1C and FIGS. 2A to 2C. FIG. 1A is an example of a cross-sectional view of a transistor 162 in a channel-length direction. FIG. 1B is an example of a cross-sectional view perpendicular to an element isolation region 165 between the transistor 162 and a transistor 163. FIG. 1C is an example of a top view of the transistor 162 and the transistor 163. Note that FIG. 1B is part of a cross-sectional view of the transistor 162 in a channel-width direction, and corresponds to a cross-sectional view taken along a dotted line D1-D2 in FIG. 1C. FIG. 1A corresponds to a cross-sectional view taken along a dotted line A1-A2 in FIG. 1C.

First, an insulating layer 130 is formed with an oxide film over a semiconductor substrate. Then, a plurality of trenches (also referred to as grooves) is formed in the insulating layer 130. Then, an oxide semiconductor layer 144 is formed so as to cover the trenches. The trenches can be formed using a known technique; in this embodiment, trenches having a depth of approximately 0.4 μm are formed. In addition, the trenches for gate electrodes are formed in a single etching step or through a plurality of etching steps.

As the semiconductor substrate, an SOI substrate, a semiconductor substrate provided with a driver circuit including a transistor with a MOSFET structure, a semiconductor substrate provided with a capacitor, or the like is used.

The insulating layer 130 is in contact with the oxide semiconductor layer 144 and therefore preferably contains a large amount of oxygen which exceeds at least the stoichiometry in (a bulk of) the layer. For example, in the case where a silicon oxide film is used as the insulating layer 130, the composition formula is SiO_(2+α) (α>0). By using the insulating layer 130 described above, oxygen can be supplied to the oxide semiconductor layer 144 and favorable characteristics can be obtained.

The oxide semiconductor layer 144 has a thickness of from 1 nm to 100 nm and contains at least one element selected from In, Ga, Sn, and Zn. As a stabilizer for reducing changes in electrical characteristics of a transistor including the oxide semiconductor, gallium (Ga) is preferably additionally contained. Tin (Sn) is preferably contained as a stabilizer. Hafnium (Hf) is preferably contained as a stabilizer. Aluminum (Al) is preferably contained as a stabilizer.

As another stabilizer, one or more lanthanoids selected from lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) may be contained.

As the oxide semiconductor, for example, an indium oxide, a tin oxide, a zinc oxide, a two-component metal oxide such as an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide, a three-component metal oxide such as an In—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide, a four-component metal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide can be used.

Note that here, for example, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Zn as its main components and there is no limitation on the ratio of In:Ga:Zn. Further, a metal element in addition to In, Ga, and Zn may be contained.

For an In—Sn—Zn-based oxide, an oxide target which has a composition ratio of In:Sn:Zn=1:2:2, 2:1:3, 1:1:1, 20:45:35, or the like in atomic ratio is used.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0, where m is not an integer) may be used as the oxide semiconductor. Note that M represents one or more metal elements selected from Ga, Fe, Mn, and Co. Alternatively, a material represented by In₃SnO₅(ZnO)_(n) (n>0, where n is an integer) may be used as the oxide semiconductor.

For example, an In—Ga—Zn-based oxide with an atomic ratio of In:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), or an oxide with an atomic ratio close to the above atomic ratios can be used. Alternatively, an In—Sn—Zn-based oxide with an atomic ratio of In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3 (=1/3:1/6:1/2), or In:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or an oxide with an atomic ratio close to the above atomic ratios may be used.

However, the composition is not limited to those described above, and a material having an appropriate composition may be used in accordance with necessary semiconductor characteristics (such as mobility, threshold voltage, and variation). In order to obtain necessary semiconductor characteristics, it is preferable that the carrier density, the impurity concentration, the defect density, the atomic ratio of a metal element to oxygen, the interatomic distance, the density, and the like be set as appropriate.

For example, with the In—Sn—Zn-based oxide, a high mobility can be relatively easily obtained. However, the mobility can be increased by reducing the defect density in the bulk also in the case of using the In—Ga—Zn-based oxide.

Note that for example, the expression “the composition of an oxide including In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1), is in the neighborhood of the composition of an oxide including In, Ga, and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b, and c satisfy the following relation: (a−A)²+(b−B)²+(c−C)²≦r², and r may be 0.05, for example. The same applies to other oxides.

The oxide semiconductor may be either single crystal or non-single-crystal. In the latter case, the oxide semiconductor may be either amorphous or polycrystalline. Further, the oxide semiconductor may have either an amorphous structure including a crystalline portion or a non-amorphous structure.

An amorphous oxide semiconductor can have a flat surface with relative ease; therefore, when a transistor is manufactured with the use of the oxide semiconductor, interface scattering can be reduced, and relatively high mobility can be obtained with relative ease.

In a crystalline oxide semiconductor, defects in the bulk can be further reduced and when a surface flatness is improved, mobility higher than that of an amorphous oxide semiconductor can be obtained. In order to improve the surface flatness, the oxide semiconductor is preferably formed over a flat surface. Specifically, the oxide semiconductor is preferably formed over a surface with an average surface roughness (R_(a)) of less than or equal to 1 nm, preferably less than or equal to 0.3 nm, more preferably less than or equal to 0.1 nm.

Note that R_(a) is obtained by three-dimensionally expanding centerline average roughness that is defined by JIS B 0601 so as to be able to be applied to a surface. R_(a) can be expressed as an “average value of the absolute values of deviations from a reference surface to a specific surface” and is defined by the following formula.

$\begin{matrix} {{Ra} = {\frac{1}{S_{0}}{\int_{y_{1}}^{y_{2}}{\int_{x_{1}}^{x_{2}}{{{{f\left( {x,y} \right)} - Z_{0}}}\ {\mathbb{d}x}\ {\mathbb{d}y}}}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack \end{matrix}$

Note that, in the above formula, S₀ represents the area of a measurement surface (a rectangular region which is defined by four points represented by the coordinates (x₁,y₁), (x₁,y₂), (x₂,y₁), and (x₂,y₂)), and Z₀ represents the average height of a measurement surface. R_(a) can be measured using an atomic force microscope (AFM).

As a material of the oxide semiconductor layer 144, a crystalline oxide semiconductor containing In and having c-axis alignment is preferably used.

In this embodiment, an oxide including a crystal with c-axis alignment, which has a triangular or hexagonal atomic arrangement when seen from the direction of an a-b plane, a surface, or an interface, will be described. In the crystal, metal atoms are arranged in a layered manner, or metal atoms and oxygen atoms are arranged in a layered manner along the c-axis, and the direction of the a-axis or the b-axis is varied in the a-b plane (the crystal rotates around the c-axis). Such a crystal is also referred to as a c-axis aligned crystal (CAAC).

An oxide including CAAC means, in a broad sense, a non-single-crystal oxide including a phase which has a triangular, hexagonal, regular triangular, or regular hexagonal atomic arrangement when seen from the direction perpendicular to the a-b plane and in which metal atoms are arranged in a layered manner or metal atoms and oxygen atoms are arranged in a layered manner when seen from the direction perpendicular to the c-axis direction.

The CAAC is not a single crystal, but this does not mean that the CAAC is composed of only an amorphous component. Although the CAAC includes a crystallized portion (crystalline portion), a boundary between one crystalline portion and another crystalline portion is not clear in some cases.

In the case where oxygen is included in the CAAC, nitrogen may be substituted for part of oxygen included in the CAAC. The c-axes of individual crystalline portions included in the CAAC may be aligned in one direction (e.g., a direction perpendicular to a surface of a substrate over which the CAAC is formed or a surface of the CAAC). Alternatively, the normals of the a-b planes of the individual crystalline portions included in the CAAC may be aligned in one direction (e.g., a direction perpendicular to a surface of a substrate over which the CAAC is formed or a surface of the CAAC).

The CAAC becomes a conductor, a semiconductor, or an insulator depending on its composition or the like. The CAAC transmits or does not transmit visible light depending on its composition or the like.

As an example of such a CAAC, there is a crystal which is formed into a film shape and has a triangular or hexagonal atomic arrangement when observed from the direction perpendicular to a surface of the film or a surface of a supporting substrate, and in which metal atoms are arranged in a layered manner or metal atoms and oxygen atoms (or nitrogen atoms) are arranged in a layered manner when a cross section of the film is observed.

An example of a crystal structure of the CAAC will be described in detail with reference to FIGS. 14A to 14E, FIGS. 15A to 15C, and FIGS. 16A to 16C. In FIGS. 14A to 14E, FIGS. 15A to 15C, and FIGS. 16A to 16C, the vertical direction corresponds to the c-axis direction and a plane perpendicular to the c-axis direction corresponds to the a-b plane, unless otherwise specified. When the expressions “an upper half” and “a lower half” are simply used, they refer to an upper half above the a-b plane and a lower half below the a-b plane (an upper half and a lower half with respect to the a-b plane). Furthermore, in FIGS. 14A to 14E, O surrounded by a circle represents tetracoordinate O and O surrounded by a double circle represents tricoordinate O.

FIG. 14A illustrates a structure including one hexacoordinate In atom and six tetracoordinate oxygen (hereinafter referred to as tetracoordinate O) atoms proximate to the In atom. Here, a structure including one metal atom and oxygen atoms proximate thereto is referred to as a small group. The structure in FIG. 14A is actually an octahedral structure, but is illustrated as a planar structure for simplicity. Note that three tetracoordinate O atoms exist in each of an upper half and a lower half in FIG. 14A. In the small group illustrated in FIG. 14A, electric charge is 0.

FIG. 14B illustrates a structure including one pentacoordinate Ga atom, three tricoordinate oxygen (hereinafter referred to as tricoordinate O) atoms proximate to the Ga atom, and two tetracoordinate O atoms proximate to the Ga atom. All the tricoordinate O atoms exist on the a-b plane. One tetracoordinate O atom exists in each of an upper half and a lower half in FIG. 14B. An In atom can also have the structure illustrated in FIG. 14B because an In atom can have five ligands. In the small group illustrated in FIG. 14B, electric charge is 0.

FIG. 14C illustrates a structure including one tetracoordinate Zn atom and four tetracoordinate O atoms proximate to the Zn atom. In FIG. 14C, one tetracoordinate O atom exists in an upper half and three tetracoordinate O atoms exist in a lower half. Alternatively, three tetracoordinate O atoms may exist in the upper half and one tetracoordinate O atom may exist in the lower half in FIG. 14C. In the small group illustrated in FIG. 14C, electric charge is 0.

FIG. 14D illustrates a structure including one hexacoordinate Sn atom and six tetracoordinate O atoms proximate to the Sn atom. In FIG. 14D, three tetracoordinate O atoms exist in each of an upper half and a lower half. In the small group illustrated in FIG. 14D, electric charge is +1.

FIG. 14E illustrates a small group including two Zn atoms. In FIG. 14E, one tetracoordinate O atom exists in each of an upper half and a lower half. In the small group illustrated in FIG. 14E, electric charge is −1.

Here, a plurality of small groups forms a medium group, and a plurality of medium groups forms a large group (also referred to as a unit cell).

Now, a rule of bonding between the small groups will be described. The three O atoms in the upper half with respect to the hexacoordinate In atom in FIG. 14A each have three proximate In atoms in the downward direction, and the three O atoms in the lower half each have three proximate In atoms in the upward direction. The one O atom in the upper half with respect to the pentacoordinate Ga atom in FIG. 14B has one proximate Ga atom in the downward direction, and the one O atom in the lower half has one proximate Ga atom in the upward direction. The one O atom in the upper half with respect to the tetracoordinate Zn atom in FIG. 14C has one proximate Zn atom in the downward direction, and the three O atoms in the lower half each have three proximate Zn atoms in the upward direction. In this manner, the number of the tetracoordinate O atoms above the metal atom is equal to the number of the metal atoms proximate to and below each of the tetracoordinate O atoms. Similarly, the number of the tetracoordinate O atoms below the metal atom is equal to the number of the metal atoms proximate to and above each of the tetracoordinate O atoms. Since the coordination number of the tetracoordinate O atom is 4, the sum of the number of the metal atoms proximate to and below the O atom and the number of the metal atoms proximate to and above the O atom is 4. Accordingly, when the sum of the number of tetracoordinate O atoms above a metal atom and the number of tetracoordinate O atoms below another metal atom is 4, the two kinds of small groups including the metal atoms can be bonded. For example, in the case where the hexacoordinate metal (In or Sn) atom is bonded through three tetracoordinate O atoms in the lower half, it is bonded to the pentacoordinate metal (Ga or In) atom or the tetracoordinate metal (Zn) atom.

A metal atom whose coordination number is 4, 5, or 6 is bonded to another metal atom through a tetracoordinate O atom in the c-axis direction. In addition to the above, a medium group can be formed in a different manner by combining a plurality of small groups so that the total electric charge of the layered structure is 0.

FIG. 15A illustrates a model of a medium group included in a layered structure of an In—Sn—Zn—O-based material. FIG. 15B illustrates a large group including three medium groups. Note that FIG. 15C illustrates an atomic arrangement in the case where the layered structure in FIG. 15B is observed from the c-axis direction.

In FIG. 15A, a tricoordinate O atom is omitted for simplicity, and a tetracoordinate O atom is illustrated by a circle; the number in the circle shows the number of tetracoordinate O atoms. For example, three tetracoordinate O atoms existing in each of an upper half and a lower half with respect to a Sn atom are denoted by circled 3. Similarly, in FIG. 15A, one tetracoordinate O atom existing in each of an upper half and a lower half with respect to an In atom is denoted by circled 1. FIG. 15A also illustrates a Zn atom proximate to one tetracoordinate O atom in a lower half and three tetracoordinate O atoms in an upper half, and a Zn atom proximate to one tetracoordinate O atom in an upper half and three tetracoordinate O atoms in a lower half.

In the medium group included in the layered structure of the In—Sn—Zn—O-based material in FIG. 15A, in the order starting from the top, a Sn atom proximate to three tetracoordinate O atoms in each of an upper half and a lower half is bonded to an In atom proximate to one tetracoordinate O atom in each of an upper half and a lower half, the In atom is bonded to a Zn atom proximate to three tetracoordinate O atoms in an upper half, the Zn atom is bonded to an In atom proximate to three tetracoordinate O atoms in each of an upper half and a lower half through one tetracoordinate O atom in a lower half with respect to the Zn atom, the In atom is bonded to a small group that includes two Zn atoms and is proximate to one tetracoordinate O atom in an upper half, and the small group is bonded to a Sn atom proximate to three tetracoordinate O atoms in each of an upper half and a lower half through one tetracoordinate O atom in a lower half with respect to the small group. A plurality of such medium groups is bonded, so that a large group is formed.

Here, electric charge for one bond of a tricoordinate O atom and electric charge for one bond of a tetracoordinate O atom can be assumed to be −0.667 and −0.5, respectively. For example, electric charge of a (hexacoordinate or pentacoordinate) In atom, electric charge of a (tetracoordinate) Zn atom, and electric charge of a (pentacoordinate or hexacoordinate) Sn atom are +3, +2, and +4, respectively. Accordingly, electric charge in a small group including a Sn atom is +1. Therefore, electric charge of −1, which cancels +1, is needed to form a layered structure including a Sn atom. As a structure having electric charge of −1, the small group including two Zn atoms as illustrated in FIG. 14E can be given. For example, with one small group including two Zn atoms, electric charge of one small group including a Sn atom can be cancelled, so that the total electric charge of the layered structure can be 0.

When the large group illustrated in FIG. 15B is repeated, an In—Sn—Zn—O-based crystal (In₂SnZn₃O₈) can be obtained. Note that a layered structure of the obtained In—Sn—Zn—O-based crystal can be expressed as a composition formula, In₂SnZn₂O₇(ZnO)_(m) (m is 0 or a natural number).

The above-described rule also applies to the following oxides: a four-component metal oxide such as an In—Sn—Ga—Zn-based oxide; a three-component metal oxide such as an In—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-based oxide, a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; a two-component metal oxide such as an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide; and the like.

As an example, FIG. 16A illustrates a model of a medium group included in a layered structure of an In—Ga—Zn—O-based material.

In the medium group included in the layered structure of the In—Ga—Zn—O-based material in FIG. 16A, in the order starting from the top, an In atom proximate to three tetracoordinate O atoms in each of an upper half and a lower half is bonded to a Zn atom proximate to one tetracoordinate O atom in an upper half, the Zn atom is bonded to a Ga atom proximate to one tetracoordinate O atom in each of an upper half and a lower half through three tetracoordinate O atoms in a lower half with respect to the Zn atom, and the Ga atom is bonded to an In atom proximate to three tetracoordinate O atoms in each of an upper half and a lower half through one tetracoordinate O atom in a lower half with respect to the Ga atom. A plurality of such medium groups is bonded, so that a large group is formed.

FIG. 16B illustrates a large group including three medium groups. Note that FIG. 16C illustrates an atomic arrangement in the case where the layered structure in FIG. 16B is observed from the c-axis direction.

Here, since electric charge of a (hexacoordinate or pentacoordinate) In atom, electric charge of a (tetracoordinate) Zn atom, and electric charge of a (pentacoordinate) Ga atom are +3, +2, and +3, respectively, electric charge of a small group including any of an In atom, a Zn atom, and a Ga atom is 0. As a result, the total electric charge of a medium group having a combination of such small groups is always 0.

In order to form the layered structure of the In—Ga—Zn—O-based material, a large group can be formed using not only the medium group illustrated in FIG. 16A but also a medium group in which the arrangement of the In atom, the Ga atom, and the Zn atom is different from that in FIG. 16A.

There are three methods for obtaining a crystalline oxide semiconductor having c-axis alignment. The first method is to form the oxide semiconductor layer 144 at a film formation temperature of 400° C. to 450° C., thereby obtaining c-axis alignment in the directions of arrows in FIG. 2A. The second method is to form a thin film and then subject the film to heat treatment performed at 200° C. to 700° C., thereby obtaining c-axis alignment in the directions of arrows in FIG. 2B. The third method is to form a first thin film, subject the film to heat treatment performed at 200° C. to 700° C., and then form a second film, thereby obtaining c-axis alignment in the directions of arrows in FIG. 2C.

As illustrated in FIGS. 2A to 2C, through any of the methods, crystal growth occurs in the direction perpendicular to an uneven surface of the oxide semiconductor layer 144, and a crystalline oxide semiconductor having c-axis alignment can be obtained.

Next, electrodes 142 a and 142 b each of which functions as a source electrode or a drain electrode are formed in contact with the oxide semiconductor layer 144. The electrodes 142 a and 142 b can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, or scandium or an alloy material which contains any of these materials as its main component.

In addition, insulating layers 143 a and 143 b are formed in order to protect the electrodes 142 a and 142 b. Next, planarization treatment is performed using chemical mechanical polishing (CMP) or the like. In this planarization treatment, the insulating layers 143 a and 143 b function as buffer layers for preventing the electrodes 142 a and 142 b from being removed.

Next, trenches for element isolation in the channel-length direction and trenches for element isolation in the channel-width direction are formed. These trenches for element isolation may have a continuous upper surface pattern shape or separate upper surface pattern shapes. In this embodiment, division of the oxide semiconductor layer is achieved by formation of the trenches; thus, these trenches have a continuous upper surface pattern shape (a lattice shape) in FIG. 1C. During the formation of the trenches for element isolation in the channel-width direction, division into the electrode 142 a and the electrode 142 b can also be achieved. Note that the timing of formation of the trenches for element isolation is not particularly limited. In addition, the depth of the trenches for element isolation is not limited to a depth at which the horizontal position of the bottoms thereof is the same as that of the bottoms of the trenches for the gate electrodes, as long as sufficient element isolation can be achieved. Element isolation can be ensured by setting the horizontal position of the bottoms of the trenches for element isolation to be deeper than that of the bottoms of the trenches for the gate electrodes.

Then, a gate insulating layer 146 is formed so as to cover part of the oxide semiconductor layer 144, the electrodes 142 a and 142 b each functioning as a source electrode or a drain electrode, and the insulating layers 143 a and 143 b. The gate insulating layer 146 is also formed on the inner walls and bottoms of the trenches for element isolation in the channel-length direction and the inner walls and bottoms of the trenches for element isolation in the channel-width direction.

The gate insulating layer 146 can be formed using a silicon oxide film, a gallium oxide film, an aluminum oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxynitride film, or a silicon nitride oxide film. A portion of the gate insulating layer 146 which is in contact with the oxide semiconductor layer 144 preferably contains oxygen. In particular, the gate insulating layer 146 preferably contains a large amount of oxygen which exceeds at least the stoichiometry in (a bulk of) the layer. For example, in the case where a silicon oxide film is used as the gate insulating layer 146, the composition formula is SiO_(2+α) (α>0). In this embodiment, a silicon oxide film of SiO_(2+α) (α>0) is used as the gate insulating layer 146. By using the silicon oxide film as the gate insulating layer 146, oxygen can be supplied to the oxide semiconductor layer 144 and favorable characteristics can be obtained. Further, the gate insulating layer 146 is preferably formed in consideration of the size of a transistor to be formed and the step coverage with the gate insulating layer 146.

When the gate insulating layer 146 is formed using a high-k material such as hafnium oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogen is added (HfSi_(x)O_(y)N_(z) (x>0, y>0, z>0)), or hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)), gate leakage current can be reduced. Further, the gate insulating layer 146 may have a single-layer structure or a stacked structure. Note that the thickness of the gate insulating layer is set greater than or equal to 1 nm and less than or equal to 100 nm.

Then, a gate electrode 148 a is formed over the gate insulating layer 146 so as to fill the trench for the gate electrode. The gate electrode 148 a can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, or scandium or an alloy material which contains any of these materials as its main component. The gate electrode 148 a may have a single-layer structure or a stacked structure.

As one layer of the gate electrode 148 a which is in contact with the gate insulating layer 146, a metal oxide containing nitrogen, specifically, an In—Ga—Zn—O film containing nitrogen, an In—Sn—O film containing nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—O film containing nitrogen, a Sn—O film containing nitrogen, an In—O film containing nitrogen, or a metal nitride (InN, SnN, or the like) film is used. These films each have a work function of 5 eV or higher, preferably 5.5 eV or higher, which enables the threshold voltage which is one of the electrical characteristics of the transistor to be positive when used as the gate electrode. Accordingly, a so-called normally off switching element can be provided.

When the gate electrode 148 a is formed in the trench for the gate electrode, the transistor 162 with a trench structure is formed.

Then, an insulating layer 149 is formed so as to cover the gate electrode 148 a and a gate electrode 148 b. As the insulating layer 149, an insulating film providing favorable step coverage is preferably used. The insulating layer 149 can be formed using a silicon oxide film, a gallium oxide film, an aluminum oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxynitride film, or a silicon nitride oxide film. In this embodiment, an aluminum oxide film is used as the insulating layer 149. In FIGS. 1A and 1B, the gate insulating layer 146 is formed in contact with a side surface of the oxide semiconductor layer 144, and furthermore, the insulating layer 149 is formed. Note that in this embodiment, a silicon oxide film of SiO_(2+α) (α>0) is used as the gate insulating layer 146, and an aluminum oxide film is used as the insulating layer 149. Accordingly, the aluminum oxide film covers the silicon oxide film, thereby blocking oxygen so as not to be diffused from the silicon oxide film and pass through the insulating layer 149.

After the insulating layer 149 is formed, an insulating layer 150 for filling the trenches for element isolation is formed by a CVD method or the like. By filling the trenches for element isolation with the insulating layer 150, element isolation regions 161 and 165 are formed. Note that when the gate insulating layer 146 and the insulating layer 149 are stacked in the trenches for element isolation before the insulating layer 150 is formed, regions to be filled with the insulating layer 150 can be smaller and can be smoothly filled with the insulating layer 150. After that, planarization treatment is performed using CMP or the like, whereby the structure illustrated in FIGS. 1A and 1B can be obtained.

As illustrated in FIG. 1B, a space between the gate electrode 148 a of the transistor 162 and the gate electrode 148 b of the transistor 163 adjacent thereto is also filled with the insulating layer 150, which makes it possible to prevent a short-circuit between the gate electrodes. Furthermore, as illustrated in FIG. 1A, a space between the electrode which functions as a source electrode or a drain electrode of the transistor 162 and an electrode which functions as a source electrode or a drain electrode of a transistor adjacent thereto in the channel-length direction is also filled with the insulating layer 150, which makes it possible to prevent a short-circuit between these electrodes.

A channel of a transistor is formed along an inner wall of a trench, and carriers smoothly flow through In—O—In—O in a CAAC oxide semiconductor even when its channel formation region is not flat. In this embodiment, the oxide semiconductor layer 144 is formed in contact with the inner wall of the trench having a depth of 0.4 μm; thus, the channel length is approximately 0.8 μm or more. A transistor with a channel length of 0.8 μm or more can be a normally off transistor, and the occurrence of short-channel effect can be prevented. In addition, by employing the trench structure, a reduction in the planar area of a transistor can be achieved, so that higher integration can be achieved.

Embodiment 2

FIGS. 3A and 3B and FIG. 17 each illustrate an example of a semiconductor device which includes the transistor 162 illustrated in FIGS. 1A to 1C, which can hold stored data even when not powered, and which has an unlimited number of write cycles.

Since the off-state current of the transistor 162 is small, stored data can be held for a long time owing to such a transistor. In other words, the frequency of refresh operation can be extremely lowered, which leads to a sufficient reduction in power consumption.

FIG. 3A illustrates an example of a cross section of the semiconductor device.

The semiconductor device illustrated in FIG. 3A includes a transistor 160 including a first semiconductor material in a lower portion, and a transistor 162 including a second semiconductor material in an upper portion. Note that the transistor 162 is the same as that in Embodiment 1; thus, for description of FIGS. 3A and 3B, the same reference numerals are used for the same parts as those in FIG. 1A.

Here, the first semiconductor material and the second semiconductor material are preferably different materials. For example, the first semiconductor material can be a semiconductor material (such as silicon) other than an oxide semiconductor, and the second semiconductor material can be an oxide semiconductor. A transistor including a material other than an oxide semiconductor can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor can hold electric charge for a long time owing to its characteristics. In this embodiment, the transistor including the first semiconductor material is referred to as a first transistor, and the transistor including the second semiconductor material is referred to as a second transistor.

Although both of the above transistors are n-channel transistors in the following description, it is needless to say that p-channel transistors can be used. The technical nature of the disclosed invention is to use an oxide semiconductor in the transistor 162 so that data can be held. Therefore, it is not necessary to limit a specific structure of the semiconductor device, such as a material of the semiconductor device or a structure of the semiconductor device, to the structure described here.

The transistor 160 in FIG. 3A includes a channel formation region 116 provided in a substrate 100 including a semiconductor material (such as silicon), impurity regions 120 provided such that the channel formation region 116 is sandwiched therebetween, metal compound regions 124 provided in contact with the impurity regions 120, a gate insulating layer 108 provided over the channel formation region 116, and a gate electrode 110 provided over the gate insulating layer 108.

An electrode 126 is connected to part of the metal compound region 124 of the transistor 160. Here, the electrode 126 functions as a source electrode or a drain electrode of the transistor 160. Further, an element isolation insulating layer (not illustrated) is formed on the substrate 100 so as to surround the transistor 160, and an insulating layer 130 is formed so as to cover the transistor 160. Note that for higher integration, it is preferable that, as in FIG. 3A, the transistor 160 does not have a sidewall insulating layer. On the other hand, when the characteristics of the transistor 160 have priority, the sidewall insulating layer may be formed on a side surface of the gate electrode 110 and the impurity regions 120 may include a region having a different impurity concentration.

As illustrated in FIG. 3A, the transistor 162 includes the oxide semiconductor layer 144 and has a trench structure.

Here, the oxide semiconductor layer 144 is preferably an oxide semiconductor layer which is purified by sufficiently removing an impurity such as hydrogen therefrom or by sufficiently supplying oxygen thereto. Specifically, the hydrogen concentration of the oxide semiconductor layer 144 is 5×10¹⁹ atoms/cm³ or less, preferably 5×10¹⁸ atoms/cm³ or less, more preferably 5×10¹⁷ atoms/cm³ or less, for example. Note that the above hydrogen concentration of the oxide semiconductor layer 144 is measured by secondary ion mass spectrometry (SIMS). The carrier concentration of the oxide semiconductor layer 144, in which hydrogen is reduced to a sufficiently low concentration so that the oxide semiconductor layer is purified and in which defect states in an energy gap due to oxygen deficiency are reduced by sufficiently supplying oxygen as described above, is less than 1×10¹²/cm³, preferably less than 1×10¹¹/cm³, more preferably less than 1.45×10¹⁰/cm³. For example, the off-state current (per unit channel width (1 μm), here) at room temperature (25° C.) is 100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A) or less, preferably 10 zA or less. In this manner, by using an i-type (intrinsic) or substantially i-type oxide semiconductor, the transistor 162 which has extremely favorable off-state current characteristics can be obtained.

Note that for the transistor 162 in FIG. 3A, an element isolation region 161 is provided in order to suppress leakage between elements due to miniaturization. Furthermore, the oxide semiconductor layer 144 which is processed in an island shape and smaller than a region surrounded by the element isolation region 161 is used; however, as described in Embodiment 1, a structure in which the oxide semiconductor layer 144 is not processed into an island shape until trenches for element isolation are formed may be employed. When the oxide semiconductor layer 144 is not processed into an island shape, the oxide semiconductor layer 144 can be prevented from being contaminated by etching during processing. It is needless to say that the number of steps can be reduced in the case where the oxide semiconductor layer 144 is not processed into an island shape. In the case of using the oxide semiconductor layer 144 which is processed in an island shape and smaller than the region surrounded by the element isolation region 161, there is no need to divide the oxide semiconductor layer by formation of trenches for element isolation, and thus, the horizontal position of the bottoms of the trenches for element isolation can be shallower than that of the bottoms of trenches for gate electrodes, or the total area of the trenches for element isolation can be reduced.

An insulating layer 151 is provided over the transistor 162, and an electrode 153 which is electrically connected to the gate electrode 148 a is provided over the insulating layer 151. In addition, an insulating layer 152 is provided over the electrode 153. An electrode 154 is provided in an opening formed in the gate insulating layer 146, the insulating layer 150, the insulating layer 151, the insulating layer 152, and the like, and a wiring 156 which is connected to the electrode 154 is formed over the insulating layer 152. Note that although the metal compound region 124, the electrode 142 b, and the wiring 156 are connected to one another through the electrode 126 and the electrode 154 in FIG. 3A, the disclosed invention is not limited thereto. For example, the electrode 142 b may be in direct contact with the metal compound region 124. Alternatively, the wiring 156 may be in direct contact with the electrode 142 b.

Next, an example of a circuit configuration corresponding to FIG. 3A is illustrated in FIG. 3B. Note that in some circuit diagrams, “OS” is written beside a transistor in order to indicate that the transistor includes an oxide semiconductor.

In FIG. 3B, a first wiring (1st Line) is electrically connected to a source electrode layer of the transistor 160. A second wiring (2nd Line) is electrically connected to a drain electrode layer of the transistor 160. A third wiring (3rd Line) is electrically connected to one of a source and a drain electrode layers of the transistor 162, and a fourth wiring (4th Line) is electrically connected to a gate electrode layer of the transistor 162. A gate electrode layer of the transistor 160 and the other of the source and drain electrode layers of the transistor 162 are electrically connected to one electrode of a capacitor 164. A fifth wiring (5th Line) is electrically connected to the other electrode of the capacitor 164.

The capacitor 164 can be formed with a pair of electrodes and an insulating layer interposed therebetween and serving as a dielectric, through the same process as the process for manufacturing the transistor 160 or the transistor 162. FIG. 17 illustrates an example in which the capacitor 164 is formed through the same process as the process for manufacturing the transistor 162. In the structure illustrated in FIG. 17, the capacitor 164 is added to the structure illustrated in FIG. 3A. After the transistor 162 is formed, an opening is formed so as to reach the source electrode 142 a of the transistor 162, and after that, an insulating film 502 is formed so as to cover the opening. Then, an electrode 504 is formed so as to be in contact with the insulating film 502 and so as to fill the opening. Thus, the capacitor 164 can be formed with the source electrode 142 a, the insulating film 502, and the electrode 504. Note that the present invention is not limited to formation of the capacitor 164 through the same process as the process for manufacturing the transistor 160 or the transistor 162, and layers of the capacitor 164 may be separately provided above the transistor 162. For example, a trench-type capacitor or a stack-type capacitor may be separately formed above the transistor 162 or below the transistor 160 so as to be three-dimensionally stacked, whereby the degree of integration may be increased.

The circuit configuration of the semiconductor device in FIG. 3B utilizes a characteristic in which the potential of the gate electrode layer of the transistor 160 can be held, and thus enables data writing, holding, and reading as follows.

Writing and holding of data will be described. First, the potential of the fourth wiring is set to a potential at which the transistor 162 is turned on, so that the transistor 162 is turned on. Accordingly, the potential of the third wiring is supplied to the gate electrode layer of the transistor 160 and to the capacitor 164. That is, predetermined charge is supplied to the gate electrode layer of the transistor 160 (writing). Here, one of two kinds of charges providing different potentials (hereinafter referred to as a low-level charge and a high-level charge) is applied. After that, the potential of the fourth wiring is set to a potential at which the transistor 162 is turned off, so that the transistor 162 is turned off. Thus, the charge supplied to the gate electrode layer of the transistor 160 is held (holding). Next, reading of data will be described. By supplying an appropriate potential (a reading potential) to the fifth wiring while supplying a predetermined potential (a constant potential) to the first wiring, the potential of the second wiring varies depending on the amount of charge held at the gate electrode layer of the transistor 160. This is because in general, when the transistor 160 is an n-channel transistor, an apparent threshold voltage V_(th) _(—) _(H) in the case where the high-level potential is given to the gate electrode layer of the transistor 160 is lower than an apparent threshold voltage V_(th) _(—) _(L) in the case where the low-level charge is given to the gate electrode layer of the transistor 160. Here, an apparent threshold voltage refers to the potential of the fifth wiring which is needed to turn on the transistor 160. Thus, the potential of the fifth wiring is set to a potential V₀ intermediate between V_(th) _(—) _(H) and V_(th) _(—) _(L), whereby charge supplied to the gate electrode layer of the transistor 160 can be determined. For example, in the case where the high-level charge is supplied in writing, when the potential of the fifth wiring is V₀ (>V_(th) _(—) _(H)), the transistor 160 is turned on. In the case where the low-level charge is supplied in writing, even when the potential of the fifth wiring is V₀ (<V_(th) _(—) _(L)), the transistor 160 remains off. Therefore, the data held can be read by measuring the potential of the second wiring.

In addition, a back gate electrode may be provided in the transistor 162, and it is preferable that the transistor 162 be surely a normally off transistor by application of the voltage to the back gate electrode.

This embodiment can be implemented in appropriate combinations with the configurations described in the other embodiments.

Embodiment 3

In this embodiment, a semiconductor device which includes the transistor 162 illustrated in FIGS. 1A to 1C, which can hold stored data even when not powered, which has an unlimited number of write cycles, and which has a structure different from the structure described in Embodiment 2 will be described with reference to FIG. 4.

The semiconductor device illustrated in FIG. 4 includes a transistor 350 including a first semiconductor material in a lower portion, and a transistor 162 including a second semiconductor material in an upper portion. Note that in this embodiment, the transistor 350 formed in the lower portion is referred to as a first transistor, and the transistor 162 formed in the upper portion is referred to as a second transistor. Although a plurality of transistors is formed in the upper and lower portions, the transistor 350 and the transistor 162 will be typically described. A plurality of transistors including the transistor 350 which is formed in the lower portion functions as a peripheral circuit of the semiconductor device, and a plurality of transistors including the transistor 162 which is formed in the upper portion functions as memory elements. Note that FIG. 4 which is taken along line B1-B2 corresponds to a cross-sectional view perpendicular to the channel-length direction of transistors.

Here, the first semiconductor material and the second semiconductor material are preferably different materials. For example, the first semiconductor material can be a semiconductor material (such as silicon) other than an oxide semiconductor, and the second semiconductor material can be an oxide semiconductor. A transistor including a material other than an oxide semiconductor can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor can hold electric charge for a long time owing to its characteristics.

Note that as illustrated in FIG. 4, the transistor 162 formed using the second semiconductor material in the upper portion includes an oxide semiconductor layer and has a trench structure. The transistor 162 is the same as the transistor 162 described in Embodiments 1 and 2; thus, for description of FIG. 4, the same reference numerals are used for the same parts as those in FIG. 1A.

The transistor 350 formed using the first semiconductor material in the lower portion will be described below.

The transistor 350 includes a semiconductor substrate 310, a gate insulating layer 314, a semiconductor layer 316, a conductive layer 318, a protective insulating layer 320, a sidewall insulating layer 322, impurity regions 324, and an insulating layer 326. Note that the semiconductor layer 316 and the conductive layer 318 function as a gate electrode, and the impurity regions 324 each function as a source region or a drain region.

In addition, the transistor 350 is adjacent to shallow trench isolation (STI) regions 312.

The STI regions 312 can be formed as follows: first, trenches (also referred to as grooves) are formed by forming a protective insulating film in a desired region over the semiconductor substrate 310 and performing etching; then, after the formation of the trenches, the trenches are filled with an insulating dielectric film. As the insulating dielectric film, a silicon oxide film, a silicon nitride film, or the like can be used.

Next, the transistor 350 will be described in detail. The gate insulating layer 314 of the transistor 350 can be formed as follows. An insulating film is formed over the semiconductor substrate 310 provided with the STI regions 312, and then, patterning and etching are performed in a desired position, whereby a trench having a depth different from that of the STI regions 312 is formed in the semiconductor substrate 310. After that, heat treatment is performed in an oxygen atmosphere, whereby the semiconductor substrate 310 in the trench is oxidized. In this manner, the gate insulating layer 314 can be formed.

After the gate insulating layer 314 is formed, a silicon film is formed using an LPCVD method or the like. Note that the silicon film is subjected to n⁺ or p⁺ doping treatment, heat treatment, or the like so as to obtain a polysilicon film, whereby a highly conductive semiconductor layer is formed. After that, a metal film is formed over the semiconductor layer by a sputtering method or the like. As the metal film, tungsten, titanium, cobalt, or nickel or an alloy film, a metal nitride film, a silicide film, or the like containing tungsten, titanium, cobalt, or nickel can be used. Patterning is performed on a desired region over the metal film, and etching is performed, whereby the conductive layer 318 is formed. In addition, the semiconductor layer is etched using the conductive layer 318 as a mask, whereby the semiconductor layer 316 can be formed. Note that the conductive layer 318 and the semiconductor layer 316 function as a gate electrode of the transistor 350.

Next, the protective insulating layer 320 is formed over the conductive layer 318. The protective insulating layer 320 can be formed in such a manner that a silicon oxide film, a silicon nitride film, or the like is formed using a plasma CVD method or the like and patterning and etching treatments are performed on a desired region.

Next, a silicon nitride film is formed using a plasma CVD method or the like so as to cover the semiconductor substrate 310 and the protective insulating layer 320 and is etched back, whereby the sidewall insulating layer 322 can be formed.

Next, the impurity regions 324 are formed by performing doping treatment using the protective insulating layer 320 and the sidewall insulating layer 322 as a mask. Note that as a dopant, boron, phosphorus, or the like may be used, and as the impurity regions 324, n⁺ regions, p⁺ regions, or the like can be formed as appropriate depending on the dopant used. Note that the impurity regions 324 each function as a source region or a drain region of the transistor 350.

Next, the insulating layer 326 is formed so as to cover the impurity regions 324, the protective insulating layer 320, and the sidewall insulating layer 322. The insulating layer 326 can be formed using a silicon oxide film or the like by a plasma CVD method or the like.

Next, openings are provided in desired regions of the insulating layer 326, and a connection electrode 325 and a connection electrode 331 are formed so as to be electrically connected to the impurity regions 324. Note that after the connection electrode 325 and the connection electrode 331 are formed, CMP treatment or the like may be performed to planarize surfaces of the insulating layer 326, the connection electrode 325, and the connection electrode 331.

Next, a conductive film is formed using a sputtering method or the like over the insulating layer 326, the connection electrode 325, and the connection electrode 331, and patterning and etching are performed on a desired region, whereby an electrode 328 and an electrode 332 are formed. As a material of the electrode 328 and the electrode 332, tungsten, copper, titanium, or the like can be used as appropriate.

Next, an insulating layer 329 is formed over the insulating layer 326, the electrode 328, and the electrode 332. The insulating layer 329 can be formed using a material and a method similar to those for the insulating layer 326.

Through the above-described process, the semiconductor substrate 310 provided with the transistor 350 including the first semiconductor material can be formed.

Next, connections between the transistor 350 including the first semiconductor material in the lower portion and the transistor 162 including the second semiconductor material in the upper portion will be described below.

The transistor 350 is electrically connected to the transistor 162 through the impurity region 324, the connection electrode 325, the electrode 328, and a connection electrode 330. On the other hand, another transistor 350 is electrically connected to the wiring 156 through the impurity region 324, the connection electrode 331, the electrode 332, a connection electrode 334, an electrode 336, and a connection electrode 338.

In addition, the gate electrode of the transistor 350 (i.e., the semiconductor layer 316 and the conductive layer 318) is electrically connected to a source electrode of the transistor 162. Note that the connection between the gate electrode of the transistor 350 and the source electrode of the transistor 162 is not illustrated in FIG. 4, and the connection is established in a three-dimensional direction.

As described above, the memory element is formed in the upper portion with the transistor including an oxide semiconductor. Since the off-state current of the transistor including an oxide semiconductor is small, stored data can be held for a long time owing to such a transistor. In other words, the frequency of refresh operation can be extremely lowered, which leads to a sufficient reduction in power consumption. On the other hand, for the peripheral circuit, a semiconductor material other than the oxide semiconductor is used. The semiconductor material other than the oxide semiconductor may be, for example, silicon, germanium, silicon germanium, silicon carbide, gallium arsenide, or the like and is preferably a single crystal semiconductor. Alternatively, an organic semiconductor material or the like may be used. A transistor including such a semiconductor material can operate at sufficiently high speed. Therefore, the transistor including the material other than the oxide semiconductor can favorably realize a variety of circuits (e.g., a logic circuit or a driver circuit) which needs to operate at high speed.

A semiconductor device having a novel feature can be obtained by being provided with both a peripheral circuit including the transistor including a material other than an oxide semiconductor (in other words, a transistor capable of operating at sufficiently high speed) and a memory circuit including the transistor including an oxide semiconductor (in a broader sense, a transistor whose off-state current is sufficiently small). In addition, with a structure where the peripheral circuit and the memory circuit are stacked, the degree of integration of the semiconductor device can be increased.

This embodiment can be implemented in appropriate combinations with the configurations described in the other embodiments.

Embodiment 4

In this embodiment, a semiconductor device which includes the transistor 162 illustrated in FIGS. 1A to 1C, which can hold stored data even when not powered, which has an unlimited number of write cycles, and which has a structure different from the structures described in Embodiments 2 and 3 will be described with reference to FIGS. 5A and 5B and FIG. 6.

FIG. 5A illustrates an example of a circuit configuration of a semiconductor device, and FIG. 5B is a conceptual diagram illustrating an example of a semiconductor device. First, the semiconductor device illustrated in FIG. 5A will be described, and then, the semiconductor device illustrated in FIG. 5B will be described.

In the semiconductor device illustrated in FIG. 5A, a bit line BL is electrically connected to a source electrode or a drain electrode of the transistor 162, a word line WL is electrically connected to a gate electrode of the transistor 162, and a source electrode or a drain electrode of the transistor 162 is electrically connected to a first terminal of a capacitor 254.

The transistor 162 including an oxide semiconductor has a characteristic of a significantly small off-state current. For that reason, a potential of the first terminal of the capacitor 254 (or a charge accumulated in the capacitor 254) can be held for an extremely long period by turning off the transistor 162. Further, in the transistor 162 including an oxide semiconductor, a short-channel effect is not likely to be caused, which is advantageous.

Next, writing and holding of data in the semiconductor device (a memory cell 250) illustrated in FIG. 5A will be described.

First, the potential of the word line WL is set to a potential at which the transistor 162 is turned on, so that the transistor 162 is turned on. Accordingly, the potential of the bit line BL is supplied to the first terminal of the capacitor 254 (writing). After that, the potential of the word line WL is set to a potential at which the transistor 162 is turned off, so that the transistor 162 is turned off. Thus, the charge at the first terminal of the capacitor 254 is held (holding).

Because the off-state current of the transistor 162 is extremely small, the potential of the first terminal of the capacitor 254 (or the charge accumulated in the capacitor) can be held for a long time.

Next, reading of data will be described. When the transistor 162 is turned on, the bit line BL which is in a floating state and the capacitor 254 are electrically connected to each other, and the charge is redistributed between the bit line BL and the capacitor 254. As a result, the potential of the bit line BL is changed. The amount of change in potential of the bit line BL varies depending on the potential of the first terminal of the capacitor 254 (or the charge accumulated in the capacitor 254).

For example, the potential of the bit line BL after charge redistribution is (C_(B)*V_(B0)+C*V)/(C_(B)+C), where V is the potential of the first terminal of the capacitor 254, C is the capacitance of the capacitor 254, C_(B) is the capacitance of the bit line BL (hereinafter also referred to as bit line capacitance), and V_(B0) is the potential of the bit line BL before the charge redistribution. Therefore, it can be found that assuming that the memory cell 250 is in either of two states in which the potentials of the first terminal of the capacitor 254 are V₁ and V₀ (V₁>V₀), the potential of the bit line BL in the case of holding the potential V₁ (=(C_(B)*V_(B0)+C*V₁)/(C_(B)+C)) is higher than the potential of the bit line BL in the case of holding the potential V₀ (=(C_(B)*V_(B0)+C*V₀)/(C_(B)+C).

Then, by comparing the potential of the bit line BL with a predetermined potential, data can be read.

As described above, the semiconductor device illustrated in FIG. 5A can hold charge that is accumulated in the capacitor 254 for a long time because the off-state current of the transistor 162 is extremely small. In other words, refresh operation becomes unnecessary or the frequency of the refresh operation can be extremely lowered, which leads to a sufficient reduction in power consumption. Moreover, stored data can be held for a long period even when power is not supplied.

Next, the semiconductor device illustrated in FIG. 5B will be described.

The semiconductor device illustrated in FIG. 5B includes a memory cell array 251 including a plurality of memory cells 250 illustrated in FIG. 5A and a memory cell array 252 including a plurality of memory cells 250 illustrated in FIG. 5A as memory elements in the upper portion, and a peripheral circuit 253 in the lower portion which is necessary for operating the memory cell array 251 and the memory cell array 252. Note that the memory cell array 252 is provided in an intermediate position between the memory cell array 251 and the peripheral circuit 253 and is provided over the peripheral circuit 253; thus, the memory cell array 251 and the memory cell array 252 are regarded as being provided in the upper portion.

In the structure illustrated in FIG. 5B, the peripheral circuit 253 can be provided under the memory cell array 251 and the memory cell array 252, and the memory cell array 251 and the memory cell array 252 can be stacked. Thus, the size of the semiconductor device can be decreased.

Next, a specific structure of the semiconductor device illustrated in FIG. 5B will be described with reference to FIG. 6.

The semiconductor device illustrated in FIG. 6 includes a plurality of memory cells (a memory cell 452 a and a memory cell 452 b) as memory elements formed in multiple layers in the upper portion, and a peripheral circuit 400 in the lower portion. The peripheral circuit 400 in the lower portion includes a transistor 450 including a first semiconductor material, and the plurality of memory cells (the memory cell 452 a and the memory cell 452 b) formed in multiple layers in the upper portion each include a transistor 162 including a second semiconductor material. The transistor 162 is connected to a capacitor, which is not illustrated in FIG. 6 because the connection is established in a three-dimensional direction. Note that FIG. 6 which is taken along line C1-C2 corresponds to a cross-sectional view perpendicular to the channel-length direction of transistors.

Here, the first semiconductor material and the second semiconductor material are preferably different materials. For example, the first semiconductor material can be a semiconductor material (such as silicon) other than an oxide semiconductor, and the second semiconductor material can be an oxide semiconductor. A transistor including a material other than an oxide semiconductor can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor can hold electric charge for a long time owing to its characteristics.

The transistor 162 including the second semiconductor material in the upper portion is the same as the transistor 162 described above in Embodiments 1 to 3; thus, for description of FIG. 6, the same reference numerals are used for the same parts as those in FIG. 1A and are not described in detail. Here, the transistor 450 including the first semiconductor material in the lower portion will be described below.

The transistor 450 in FIG. 6 includes a channel formation region 404 provided in a substrate 402 including a semiconductor material (such as silicon), impurity regions 406 and high-concentration impurity regions 408 (collectively, simply also referred to as impurity regions) provided such that the channel formation region 404 is sandwiched therebetween, metal compound regions 410 provided in contact with the high-concentration impurity regions 408, a gate insulating layer 411 provided over the channel formation region 404, a gate electrode layer 412 provided in contact with the gate insulating layer 411, and a source or drain electrode 418 a and a source or drain electrode 418 b electrically connected to the impurity regions.

Here, a sidewall insulating layer 414 is provided on a side surface of the gate electrode layer 412. Further, an element isolation insulating layer 403 is formed on the substrate 402 so as to surround the transistor 450, and an interlayer insulating layer 420 and an interlayer insulating layer 422 are formed so as to cover the transistor 450. The source or drain electrode 418 a and the source or drain electrode 418 b are electrically connected to the metal compound regions 410 through openings formed in the interlayer insulating layer 420 and the interlayer insulating layer 422. In other words, the source or drain electrode 418 a and the source or drain electrode 418 b are electrically connected to the high-concentration impurity regions 408 and the impurity regions 406 through the metal compound regions 410. Note that in some cases, the sidewall insulating layer 414 is not formed, in order to achieve a higher degree of integration of the transistor 450 or the like. In addition, an electrode 424 a, an electrode 424 b, and an electrode 424 c which are electrically connected to the source or drain electrode 418 a and the source or drain electrode 418 b of the transistor 450 are provided over the interlayer insulating layer 422, and planarization is achieved with an insulating layer 425 which covers the interlayer insulating layer 422, the electrode 424 a, the electrode 424 b, and the electrode 424 c.

The electrode 424 c is electrically connected to an electrode 428 through a connection electrode 426. Note that the electrode 428 is formed using the same layer as the source electrode layer and the drain electrode layer of the transistor 162.

In addition, a wiring 432 is electrically connected to the electrode 428 through a connection electrode 430 and is electrically connected to an electrode 436 which is formed using the same layer as the source electrode layer and the drain electrode layer of the transistor 162, through a connection electrode 434. In addition, the electrode 436 is electrically connected to a wiring 440 through a connection electrode 438.

With the electrode 424 c, the wiring 432, and the wiring 440, an electrical connection between memory cells, an electrical connection between the peripheral circuit 400 and memory cells (the memory cell 452 a and the memory cells 452 b), or the like can be established.

Note that FIG. 6 illustrates, as an example, the semiconductor device in which two memory cells (the memory cell 452 a and the memory cell 452 b) are stacked; however, the number of memory cells to be stacked is not limited thereto. Three or more memory cells may be stacked.

In addition, FIG. 6 illustrates, as an example, the semiconductor device in which the memory cell 452 a, the memory cell 452 b, and the peripheral circuit 400 are connected through the electrode 424 c, the electrode 428, the wiring 432, the electrode 436, and the wiring 440; however, the present invention is not limited thereto. Two or more wiring layers and electrodes may be provided between the memory cell 452 a, the memory cell 452 b, and the peripheral circuit 400.

As described above, the plurality of memory cells formed in multiple layers in the upper portion is each formed with a transistor including an oxide semiconductor. Since the off-state current of the transistor including an oxide semiconductor is small, stored data can be held for a long time owing to such a transistor. In other words, the frequency of refresh operation can be extremely lowered, which leads to a sufficient reduction in power consumption. On the other hand, for the peripheral circuit, a semiconductor material other than the oxide semiconductor is used. The semiconductor material other than the oxide semiconductor may be, for example, silicon, germanium, silicon germanium, silicon carbide, gallium arsenide, or the like and is preferably a single crystal semiconductor. Alternatively, an organic semiconductor material or the like may be used. A transistor including such a semiconductor material can operate at sufficiently high speed. Therefore, the transistor including the material other than the oxide semiconductor can favorably realize a variety of circuits (e.g., a logic circuit or a driver circuit) which needs to operate at high speed.

A semiconductor device having a novel feature can be obtained by being provided with both a peripheral circuit including the transistor including a material other than an oxide semiconductor (in other words, a transistor capable of operating at sufficiently high speed) and a memory circuit including the transistor including an oxide semiconductor (in a broader sense, a transistor whose off-state current is sufficiently small). In addition, with a structure where the peripheral circuit and the memory circuit are stacked, the degree of integration of the semiconductor device can be increased.

This embodiment can be implemented in appropriate combinations with the configurations described in the other embodiments.

Embodiment 5

In this embodiment, examples of application of the semiconductor device described in any of the above embodiments to portable devices such as cellular phones, smartphones, or electronic books will be described with reference to FIGS. 10A and 10B and FIGS. 11 to 13.

In a portable device such as a cellular phone, a smartphone, or an electronic book, an SRAM or a DRAM is used so as to store image data temporarily. The reason why an SRAM or a DRAM is used is that a flash memory is slow in responding and is not suitable for image processing. On the other hand, an SRAM or a DRAM has the following characteristics when used for temporary storage of image data.

In an ordinary SRAM, as illustrated in FIG. 10A, one memory cell includes six transistors, that is, transistors 801 to 806, which are driven with an X decoder 807 and a Y decoder 808. The transistor 803 and the transistor 805, and the transistor 804 and the transistor 806 form inverters, which enables high-speed driving. However, because one memory cell includes six transistors, a large cell area is one disadvantage. Provided that the minimum feature size of a design rule is F, the area of a memory cell in an SRAM is generally 100 F² to 150 F². Therefore, the price per bit of an SRAM is the most expensive among memory devices.

In a DRAM, as illustrated in FIG. 10B, a memory cell includes a transistor 811 and a storage capacitor 812, which are driven with an X decoder 813 and a Y decoder 814. One cell is configured with one transistor and one capacitor and has a small area. The area of a memory cell in a DRAM is generally 10 F² or less. Note that the DRAM needs to be refreshed periodically and consumes electric power even when a rewriting operation is not performed.

On the other hand, the memory cell of the semiconductor device described in any of the above embodiments has an area of approximately 10 F² and does not need to be refreshed frequently. Therefore, the area of a memory cell can be decreased, and power consumption can be reduced.

Next, FIG. 11 is a block diagram of a portable device. The portable device illustrated in FIG. 11 includes an RF circuit 901, an analog baseband circuit 902, a digital baseband circuit 903, a battery 904, a power supply circuit 905, an application processor 906, a flash memory 910, a display controller 911, a memory circuit 912, a display 913, a touch sensor 919, an audio circuit 917, a keyboard 918, and the like. The display 913 includes a display portion 914, a source driver 915, and a gate driver 916. The application processor 906 includes a CPU 907, a DSP 908, and an interface 909 (IF 909). In general, the memory circuit 912 includes an SRAM or a DRAM. By employing the semiconductor device described in any of the above embodiments for that portion, data can be written and read at high speed and can be held for a long time, and power consumption can be sufficiently reduced.

Next, FIG. 12 illustrates an example of using the semiconductor device described in any of the above embodiments in a memory circuit 950 for a display. The memory circuit 950 illustrated in FIG. 12 includes a memory 952, a memory 953, a switch 954, a switch 955, and a memory controller 951. The memory circuit 950 is connected to a display controller 956 that reads and controls image data input through a signal line (input image data) and data stored in the memory 952 and the memory 953 (stored image data), and is also connected to a display 957 that displays an image based on a signal input from the display controller 956.

First, image data (input image data A) is produced by an application processor (not illustrated). The input image data A is stored in the memory 952 through the switch 954. Then, the image data stored in the memory 952 (stored image data A) is transmitted to the display 957 through the switch 955 and the display controller 956, and is displayed on the display 957.

When the input image data A remains unchanged, the stored image data A is read from the memory 952 through the switch 955 by the display controller 956 normally at a frequency of approximately 30 Hz to 60 Hz.

Next, for example, when a user performs an operation to rewrite a screen (i.e., when the input image data A is changed), the application processor produces new image data (input image data B). The input image data B is stored in the memory 953 through the switch 954. Also during that time, the stored image data A is regularly read from the memory 952 through the switch 955. After the completion of storing the new image data (the stored image data B) in the memory 953, from the next frame for the display 957, the stored image data B starts to be read, transmitted to the display 957 through the switch 955 and the display controller 956, and displayed on the display 957. This reading operation continues until the next new image data is stored in the memory 952.

By alternately writing and reading image data to and from the memory 952 and the memory 953 as described above, images are displayed on the display 957. Note that the memory 952 and the memory 953 are not limited to separate memories, and a single memory may be divided and used. By employing the semiconductor device described in any of the above embodiments for the memory 952 and the memory 953, data can be written and read at high speed and held for a long time, and power consumption can be sufficiently reduced.

Next, FIG. 13 is a block diagram of an electronic book. FIG. 13 includes a battery 1001, a power supply circuit 1002, a microprocessor 1003, a flash memory 1004, an audio circuit 1005, a keyboard 1006, a memory circuit 1007, a touch panel 1008, a display 1009, and a display controller 1010.

Here, the semiconductor device described in any of the above embodiments can be used for the memory circuit 1007 in FIG. 13. The memory circuit 1007 has a function to temporarily hold the contents of a book. For example, a user may use a highlight function. In some cases, a user wants to mark a specific portion while reading an electronic book. This marking function is called highlight function and is used to make a difference from the other portions by changing the display color, underlining, making characters bold, changing the font of characters, or the like. The function makes it possible to store and hold data of a portion specified by a user. In order to store the data for a long time, the data may be copied to the flash memory 1004. Also in such a case, by employing the semiconductor device described in any of the above embodiments, data can be written and read at high speed and held for a long time, and power consumption can be sufficiently reduced.

As described above, the portable devices described in this embodiment each incorporates the semiconductor device according to any of the above embodiments. Therefore, it is possible to obtain a portable device which is capable of reading data at high speed, holding data for a long time, and reducing power consumption.

The configurations, methods, and the like described in this embodiment can be combined as appropriate with any of the configurations, methods, and the like described in the other embodiments.

Example 1

In this example, calculations were carried out to determine whether or not a short-channel effect is caused in the transistor having a trench structure which is described in Embodiment 1. FIGS. 7A and 7B, FIGS. 8A and 8B, and FIGS. 9A and 9B show structures used for the calculations and results of the calculations. Note that in each of FIGS. 7A, 8A, and 9A, a gate insulating layer is denoted by SiO_(x), an oxide semiconductor layer is denoted by OS, a gate electrode is denoted by Gate, and a source electrode and a drain electrode are denoted by S/D.

Note that for the calculations, device simulation software Sentaurus Device manufactured by Synopsys, Inc. was used.

FIG. 7A shows a structure used for the calculation and the sizes of components. The thickness of the gate insulating layer is set to 5 nm, the thickness of the oxide semiconductor layer is set to 5 nm, and the depth of the trench for the gate electrode is set to 0.4 μm. FIG. 7A shows a transistor having a trench structure in which the length of the bottom of the trench (the length in the channel-length direction) is 90 nm and the distance between the source electrode and the drain electrode (the length in the channel-length direction) is 110 nm A material of the oxide semiconductor layer is an In—Ga—Zn—O-based oxide semiconductor (with a band gap of 3.15 eV, an electron affinity of 4.6 eV, and an electron mobility of 10 cm²/Vs), the work function of the electrodes in contact with the oxide semiconductor layer (the source electrode and the drain electrode) is 4.6 eV, and the work function of the gate electrode is 5.5 eV. FIG. 7B shows the result of a calculation of Vg-Id characteristics of the transistor having the trench structure (with Vds=1 V at a temperature of 27° C.).

FIG. 8A shows a transistor having a trench structure in which the length of the bottom of the trench (the length in the channel-length direction) is 60 nm and the distance between the source electrode and the drain electrode (the length in the channel-length direction) is 80 nm FIG. 8B shows the result of a calculation carried out with the same conditions as in FIG. 7B except the length of the bottom of the trench and the distance between the source electrode and the drain electrode.

FIG. 9A shows a transistor having a trench structure in which the length of the bottom of the trench (the length in the channel-length direction) is 30 nm and the distance between the source electrode and the drain electrode (the length in the channel-length direction) is 50 nm FIG. 9B shows the result of a calculation carried out with the same conditions as in FIG. 7B except the length of the bottom of the trench and the distance between the source electrode and the drain electrode.

The results of the calculations show that all the transistors having the structures in FIGS. 7A, 8A, and 9A have substantially the same characteristics. The threshold voltage (Vth) of each transistor is 0.8 V and the subthreshold swing (S value) thereof is 60 mV/dec, which are favorable values.

These calculation results reveal that a short-channel effect such as a negative shift of the threshold voltage or an increase in the subthreshold swing is not caused even when the distance between the source electrode and the drain electrode (the length in the channel-length direction) is decreased to 30 nm, and favorable transistor characteristics are obtained.

For comparison, similar calculations were carried out using transistors having not a trench structure but a planar structure. As the distance between the source electrode and the drain electrode (the length in the channel-length direction) decreased, the channel length also decreased. A short-channel effect such as a negative shift of the threshold voltage or an increase in the subthreshold swing was caused. Furthermore, an increase in leakage current (off-state current) generated when a negative bias was applied to the gate was also observed.

Compared with the results of the comparative calculations, the results of the calculations in FIGS. 7B, 8B, and 9B are favorable. With the transistor structure described in Embodiment 1, the change in substantial channel length is small even when the distance between the source electrode and the drain electrode (the length in the channel-length direction) is decreased. Therefore, a short-channel effect is not caused, and off-state current can be small. Accordingly, a memory cell having favorable retention characteristics can be produced.

EXPLANATION OF REFERENCE

100: substrate, 108: gate insulating layer, 110: gate electrode, 116: channel formation region, 120: impurity region, 124: metal compound region, 126: electrode, 130: insulating layer, 142 a: electrode, 142 b: electrode, 143 a: insulating layer, 143 b: insulating layer, 144: oxide semiconductor layer, 146: gate insulating layer, 148 a: gate electrode, 148 b: gate electrode, 149: insulating layer, 150: insulating layer, 151: insulating layer, 152: insulating layer, 153: electrode, 154: electrode, 156: wiring, 160: transistor, 161: element isolation region, 162: transistor, 163: transistor, 164: capacitor, 165: element isolation region, 250: memory cell, 251: memory cell array, 252: memory cell array, 253: peripheral circuit, 254: capacitor, 310: semiconductor substrate, 312: STI region, 314: gate insulating layer, 316: semiconductor layer, 318: conductive layer, 320: protective insulating layer, 322: sidewall insulating layer, 324: impurity region, 325: connection electrode, 326: insulating layer, 328: electrode, 329: insulating layer, 330: connection electrode, 331: connection electrode, 332: electrode, 334: connection electrode, 336: electrode, 338: connection electrode, 350: transistor, 400: peripheral circuit, 402: substrate, 403: element isolation insulating layer, 404: channel formation region, 406: impurity region, 408: high-concentration impurity region, 410: metal compound region, 411: gate insulating layer, 412: gate electrode layer, 414: sidewall insulating layer, 418 a: source or drain electrode, 418 b: source or drain electrode, 420: interlayer insulating layer, 422: interlayer insulating layer, 424 a: electrode, 424 b: electrode, 424 c: electrode, 425: insulating layer, 426: connection electrode, 428: electrode, 430: connection electrode, 432: wiring, 434: connection electrode, 436: electrode, 438: connection electrode, 440: wiring, 450: transistor, 452 a: memory cell, 452 b: memory cell, 502: insulating film, 504: electrode, 801: transistor, 803: transistor, 804: transistor, 805: transistor, 806: transistor, 807: X decoder, 808: Y decoder, 811: transistor, 812: storage capacitor, 813: X decoder, 814: Y decoder, 901: RF circuit, 902: analog baseband circuit, 903: digital baseband circuit, 904: battery, 905: power supply circuit, 906: application processor, 907: CPU, 908: DSP, 909: interface, 910: flash memory, 911: display controller, 912: memory circuit, 913: display, 914: display portion, 915: source driver, 916: gate driver, 917: audio circuit, 918: keyboard, 919: touch sensor, 950: memory circuit, 951: memory controller, 952: memory, 953: memory, 954: switch, 955: switch, 956: display controller, 957: display, 1001: battery, 1002: power supply circuit, 1003: microprocessor, 1004: flash memory, 1005: audio circuit, 1006: keyboard, 1007: memory circuit, 1008: touch panel, 1009: display, and 1010: display controller.

This application is based on Japanese Patent Application serial no. 2011-014626 filed with Japan Patent Office on Jan. 26, 2011 and Japanese Patent Application serial no. 2011-112675 filed with Japan Patent Office on May 19, 2011, the entire contents of which are hereby incorporated by reference. 

The invention claimed is:
 1. A semiconductor device comprising: a circuit comprising a first transistor; and a memory element comprising a second transistor and a capacitor, wherein the second transistor comprises: a first insulating layer; a first trench in the first insulating layer; a second trench in the first insulating layer; an oxide semiconductor layer in contact with an inner wall surface of the first trench; a second insulating layer adjacent to the oxide semiconductor layer, wherein the second insulating layer is adjacent to an inner wall surface of the second trench; a gate electrode in the first trench and adjacent to the oxide semiconductor layer with the second insulating layer interposed therebetween; a third insulating layer filling the second trench; and a source electrode or a drain electrode in contact with the oxide semiconductor layer, and wherein the memory element is stacked over the circuit.
 2. The semiconductor device according to claim 1, wherein a gate electrode of the first transistor is electrically connected to one of a pair of electrodes included in the capacitor and the source electrode or the drain electrode in the second transistor.
 3. The semiconductor device according to claim 1, wherein the second insulating layer is in contact with a side surface of the oxide semiconductor layer.
 4. The semiconductor device according to claim 1, wherein the oxide semiconductor layer includes a crystal having a c-axis substantially perpendicular to a surface of the oxide semiconductor layer.
 5. The semiconductor device according to claim 1, wherein the first transistor comprises a semiconductor material other than an oxide semiconductor material, and wherein the semiconductor material is selected from the group consisting of silicon, germanium, silicon germanium, silicon carbide, and gallium arsenide.
 6. The semiconductor device according to claim 1, further comprising a fourth insulating layer in the second trench, wherein the fourth insulating layer is provided between the second insulating layer and the third insulating layer.
 7. A semiconductor device comprising: a semiconductor substrate provided with a first transistor; a second transistor over the first transistor, the second transistor comprising: a first insulating layer; a first trench in the first insulating layer; a first oxide semiconductor layer in contact with an inner wall surface of the first trench; a second insulating layer adjacent to the first oxide semiconductor layer; and a first gate electrode in the first trench and adjacent to the first oxide semiconductor layer with the second insulating layer interposed therebetween; and a third transistor over the second transistor, the third transistor comprising: a third insulating layer; a second trench in the third insulating layer; a second oxide semiconductor layer in contact with an inner wall surface of the second trench; a fourth insulating layer adjacent to the second oxide semiconductor layer; and a second gate electrode in the second trench and adjacent to the second oxide semiconductor layer with the fourth insulating layer interposed therebetween.
 8. The semiconductor device according to claim 7, wherein the second insulating layer is in contact with a side surface of the first oxide semiconductor layer, and wherein the fourth insulating layer is in contact with a side surface of the second oxide semiconductor layer.
 9. The semiconductor device according to claim 7, wherein the first transistor and the second transistor are electrically connected to each other, and wherein the second transistor and the third transistor are electrically connected to each other.
 10. The semiconductor device according to claim 7, further comprising a first pair of electrodes in contact with the first oxide semiconductor layer, wherein a gate electrode of the first transistor and one of the first pair of electrodes are electrically connected to each other.
 11. The semiconductor device according to claim 10, further comprising a second pair of electrodes in contact with the second oxide semiconductor layer, wherein the other of the first pair of electrodes and one of the second pair of electrodes are electrically connected to each other.
 12. The semiconductor device according to claim 10, further comprising a first capacitor over the second transistor, wherein the first capacitor is electrically connected to the first transistor and the second transistor.
 13. The semiconductor device according to claim 10, further comprising a second capacitor over the third transistor, wherein the second capacitor is electrically connected to the third transistor.
 14. The semiconductor device according to claim 7, wherein the first oxide semiconductor layer includes a crystal having a c-axis substantially perpendicular to a surface of the first oxide semiconductor layer.
 15. The semiconductor device according to claim 7, wherein the second oxide semiconductor layer includes a crystal having a c-axis substantially perpendicular to a surface of the second oxide semiconductor layer.
 16. The semiconductor device according to claim 7, wherein the first transistor comprises a semiconductor material, and wherein the semiconductor material is selected from the group consisting of silicon, germanium, silicon germanium, silicon carbide, and gallium arsenide.
 17. The semiconductor device according to claim 7, further comprising a third trench in the first insulating layer, wherein the second insulating layer is adjacent to an inner wall surface of the third trench, and wherein a fifth insulating layer fills the third trench.
 18. The semiconductor device according to claim 17, further comprising a fourth trench in the third insulating layer, wherein the fourth insulating layer is adjacent to an inner wall surface of the fourth trench, and wherein a sixth insulating layer fills the fourth trench.
 19. A semiconductor device comprising: a semiconductor substrate provided with a first transistor; and a second transistor over the first transistor, the second transistor comprising: a first insulating layer; a first trench in the first insulating layer; a first oxide semiconductor layer in contact with the first insulating layer, wherein the first oxide semiconductor layer comprising: a first region adjacent to a first side wall of the first trench; a second region adjacent to a bottom surface of the first trench; and a third region adjacent to a second side wall of the first trench, the first side wall of the first trench being opposite to the second side wall of the first trench; a first source electrode on a first region of the first insulating layer, the first source electrode being in electrical contact with the first oxide semiconductor layer; a first drain electrode on a second region of the first insulating layer, the first drain electrode being in electrical contact with the first oxide semiconductor layer, wherein the first trench is located between the first region of the first insulating layer and the second region of the first insulating layer; a first gate insulating layer adjacent to the first oxide semiconductor layer; and a first gate electrode in the first trench and adjacent to the first oxide semiconductor layer with the first gate insulating layer interposed therebetween.
 20. The semiconductor device according to claim 19, further comprising a third transistor over the second transistor, the third transistor comprising: a second insulating layer; a second trench in the second insulating layer; a second oxide semiconductor layer in contact with the second insulating layer, wherein the second oxide semiconductor layer comprising: a fourth region adjacent to a first side wall of the second trench; a fifth region adjacent to a bottom surface of the second trench; and a sixth region adjacent to a second side wall of the second trench, the first side wall of the second trench being opposite to the second side wall of the second trench; a second source electrode on a first region of the second insulating layer, the second source electrode being in electrical contact with the second oxide semiconductor layer; a second drain electrode on a second region of the second insulating layer, the second drain electrode being in electrical contact with the second oxide semiconductor layer, wherein the second trench is located between the first region of the second insulating layer and the second region of the second insulating layer; a second gate insulating layer adjacent to the second oxide semiconductor layer; and a second gate electrode in the second trench and adjacent to the second oxide semiconductor layer with the second gate insulating layer interposed therebetween.
 21. The semiconductor device according to claim 19, wherein the first gate insulating layer is in contact with a side surface of the first oxide semiconductor layer.
 22. The semiconductor device according to claim 19, wherein a gate electrode of the first transistor and the first source electrode are electrically connected to each other.
 23. The semiconductor device according to claim 19, further comprising a capacitor over the second transistor, wherein the capacitor is electrically connected to the first transistor and the second transistor.
 24. The semiconductor device according to claim 19, wherein the first oxide semiconductor layer includes a crystal having a c-axis substantially perpendicular to a surface of the first oxide semiconductor layer.
 25. The semiconductor device according to claim 19, wherein the first transistor comprises a semiconductor material other than an oxide semiconductor material, and wherein the semiconductor material is selected from the group consisting of silicon, germanium, silicon germanium, silicon carbide, and gallium arsenide.
 26. The semiconductor device according to claim 19, further comprising: a third trench in the first insulating layer; and a third insulating layer filling the third trench.
 27. The semiconductor device according to claim 26, further comprising a fourth insulating layer in the third trench, wherein the fourth insulating layer is provided between the first insulating layer and the third insulating layer. 